Biaxially-oriented polyolefin multi-layer film which can be sealed on both sides and the preparation and use of the same

ABSTRACT

A three layer, biaxially-oriented polyolefin film which is sealable on both exterior surfaces and includes a base layer of polypropylene which carries two sealing layers of sealable olefin polymers. The base layer includes about 0.5 to 2% by weight of polydialkylsiloxane having a viscosity of less than about 500 mm 2  /s. The film is particularly well-suited for the reception of aqueous printing inks or as a substrate for aqueous coating systems.

BACKGROUND OF THE INVENTION

The present invention relates to a biaxially-oriented polyolefin filmwhich can be sealed on both sides and is comprised of three layers, thebase layer consisting essentially of propylene polymers and the twosealing layers consisting essentially of sealable olefin polymers, andwhich is distinguished by universal applicability to high-speedpackaging machines and is also readily printable. The present inventionalso relates to a process for the preparation of the film and to itsuse.

EP-A-0 008 904 discloses a biaxially-oriented, three-layer polyolefinfilm which can be sealed on both sides and in which the base layer isformed by propylene polymers and the two sealing layers, i.e., the outeror top layers, are formed by sealable olefin polymers. Although thispolyolefin film has good heat-sealability, it has, in particular, only alow scratch resistance, cannot be printed and also leaves much to bedesired with respect to transparency and slip properties in conjunctionwith high-speed packaging machines.

German Offenlegungsschrift 3,247,998 discloses a biaxially-orientedpolyolefin film consisting of three layers which is transparent andparticularly readily sealable. Its base layer consists essentially of apropylene polymer, and the two sealing layers consist of an appropriateolefin polymer containing, as additives, 5 to 15% by weight of alow-molecular resin which is compatible with the olefin polymer, 5 to15% by weight of a propylene homopolymer and 0.3 to 1.5% by weight ofpolydiorganosiloxane, the percentages by weight relating in each case tothe sealing layer. This polyolefin multi-layer film possessescharacteristics which are important for packaging films, namely a widerange of sealing, a low sealing temperature, high gloss and goodtransparency. The film also displays a relatively high scratchresistance and low friction and hence good running characteristics onhigh-speed packaging machines of various types. However, it lacksanother characteristic which is also important, namely goodprintability.

A biaxially-oriented polyolefin film which can be sealed on both sidesand comprises three layers is also disclosed in U.S. Pat. No. 4,419,411.In this film, an additive combination is also incorporated in both ofthe sealing layers. They each contain 0.15 to 0.4% by weight ofpolysiloxane and 0.05 to 0.5% by weight of silicon dioxide, thepercentages by weight relating in each case to the sealable layer. Thebase layer is made of polypropylene (as the main component) and containsa small amount of a monocarboxamide. Part of the amide migrates from thebase layer into the two sealing layers and onto the surfaces thereof(external faces), so that polysiloxane, silicon dioxide and themonocarboxamide are present in incorporated form in each sealing layer,and the monocarboxamide is also present on the outer faces of the twosealing layers. The polyolefin multi-layer film described is stated tohave a particularly low coefficient of friction. However, this film alsosuffers in particular from the disadvantage that it is not printable.

SUMMARY OF THE INVENTION

It is therefore an object of the instant invention to provide abiaxially-oriented polyolefin film which can be sealed on both exteriorsurfaces and is comprised of three layers and which has the importantcharacteristics mentioned above and, in addition, is also readilyprintable or coatable. The new polyolefin multi-layer film should, inparticular, be readily sealable on both exterior surfaces and shouldhave excellent running characteristics on packaging machines, and itshould furthermore also be printable or coatable on one or both exteriorsurfaces. Another object of the present invention is to provide aprocess for producing the above-described film and a packaging materialwhich incorporates the film.

In accomplishing the foregoing objects there is provided according tothe present invention a three layer, biaxially-oriented polyolefin filmhaving two exterior surfaces which can be sealed, comprising a baselayer comprised of propylene and having a first and second surface, andfirst and second sealing layers comprised of a sealable olefin polymerand disposed on said first and second surfaces, respectively, of saidbase layer, wherein the base layer includes about 0.5 to 2% by weight,preferably 0.6 to 1.5% by weight, most preferably 0.7 to 1.0% by weight,relative to the weight of the layer, of a polydialkylsiloxane having aviscosity of less than about 500 mm² /s, in particular of between 100and 10 mm² /s. Also provided according to the present invention is aprocess for producing the above-described film and a packaging materialwhich incorporates the film.

Further objects, features and advantages of the present invention willbecome apparent from the detailed description of preferred embodimentsthat follows.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The polydialkylsiloxane incorporated in the base layer preferably has 1to 4 carbon atoms in its alkyl groups and a viscosity, at about 25° C.,of less than about 500 mm² /s, preferably of less than about 200 mm² /s,in particular of about 10 to 100 mm² /s. Particular preference is givento polydimethylsiloxane. In the case of polydimethylsiloxane, theviscosity range of, for example, about 10 to 100 mm² /s at about 25° C.corresponds to a mean molecular weight of about 1,200 to 5,700 g/mol.

The base layer of the multi-layer film according to the presentinvention is comprised of a propylene polymer which is madepredominantly of propylene and has a melting point of about 140° C. orhigher, preferably of about 150° to 170° C. Isotactic polypropylenehaving an n-heptane-soluble proportion of about 15% by weight or less,copolymers of ethylene and propylene having an ethylene content of about10% by weight or less, and copolymers of propylene and C₄ to C₈α-olefins having an α-olefin content of about 10% by weight or lessrepresent preferred propylene polymers for the base layer, isotacticpolypropylene being particularly preferred. In general, the propylenepolymer of the base layer has a melt flow index of about 0.5 g/10minutes to 10 g/10 minutes, preferably of about 1.5 g/10 minutes to 4g/10 minutes, determined at about 230° C. and at a loading of about21.6N in accordance with DIN 53 735.

The sealing layers of the multi-layer film according to the presentinvention are comprised of a sealable olefinic polymer. Advantageousolefin polymers are ethylene homopolymers, copolymers formed fromethylene and propylene or from ethylene or propylene and butene-1 oranother α-olefin having 5 to 10 carbon atoms, terpolymers formed fromethylene, propylene and butene-1 or another α-olefin having 5 to 10carbon atoms, or mixtures of these polymers. It is preferable to employethylene/propylene copolymers, ethylene/butylene copolymers,propylene/butylene copolymers, ethylene/propylene/butylene terpolymersor mixtures of these polymers. Olefin polymers which are particularlypreferred for the sealing layers are ethylene/propylene copolymershaving propylene as the main component and an ethylene content of about2 to 10% by weight (relative to the copolymer), propylene/butylenecopolymers having propylene as the main component and a butylene contentof about 0.5 to 25% by weight (relative to the copolymer) andethylene/propylene/butylene terpolymers having propylene as the maincomponent, about 0.5 to 7% by weight of ethylene and about 5 to 30% byweight of butylene (the percentages by weight relate to the terpolymer)and also mixtures of these polymers.

The olefin polymer of the sealing layers has a lower melting point thanthe propylene polymer of the base layer. The melting point of the olefinpolymer is, in general, within about 80° to 160° C., preferably fromabout 100° to 140° C. The melt flow index of the olefin polymer of thesealing layers is higher than that of the propylene polymer of the baselayer. The olefin polymer employed for the sealing layers has a meltflow index of, in general, about 1 to 12 g/10 minutes, preferably ofabout 3 to 9 g/10 minutes, measured at about 230° C. and a loading of21.6N in accordance with DIN 53 735.

Any surface treatment of the sealing layers which may be desired, suchas a flame or corona treatment, is performed following the laststretching stage. For the corona treatment, which can be carried out byany of the known methods, it is expedient to use a procedure in whichthe film is passed between two conductor elements acting as electrodes,and to apply, between the electrodes, a voltage, in most cases analternating voltage, sufficiently high (about 10,000 volt and 10,000Hertz) to enable spray or corona discharges to take place. As a resultof the spray or corona discharges, the air above the surface of the filmis ionized and combines with the molecules on the surface of the film,so that polar incorporations are formed in the essentially nonpolarpolymer matrix. In accordance with the present invention, one or bothsealing layers can be subjected to corona treatment. The film of thisinvention has particularly good adhesive properties in respect ofaqueous coating compositions.

In order to improve even further certain properties of the polyolefinfilm according to the present invention, it is possible for the baselayer and the two sealing layers to include an effective amount ofadditives, preferably antistatic agents, antiblocking agents, slipagents, fillers, pigments, dyes, stabilizers and/or low-molecularresins, which are compatible with the polymer of the base layer and ofthe sealing layers.

Preferred antistatic agents are saturated aliphatic, tertiary aminescontaining an aliphatic radical having 10 to 20 carbon atoms andsubstituted by 2-hydroxyalkyl-(C₁ -C₄) groups, amongst whichN,N-bis-(2-hydroxyethyl)alkylamines having C₁₀ -C₂₀ groups, preferablyC₁₂ -C₁₈ groups, as the alkyl groups are particularly suitable. Theeffective amount of antistatic agent is within about 0.05 to 3% byweight, relative to the weight of the respective layer.

Advantageous antiblocking agents, which preferably are added to the toplayers, include inorganic additives, such as silicon dioxide, calciumcarbonate, magnesium silicate, aluminum silicate, calcium phosphate andthe like, nonionic surfactants, anionic surfactants and/or incompatibleorganic polymers, such as polyamides, polyesters, polycarbonates and thelike. The effective amount of anti-blocking agent is within about 0.1 to2% by weight, relative to the layer.

Examples of slip agents are higher aliphatic acid amides, higheraliphatic acid esters, waxes and metal soaps. The effective amount ofslip agent is within about 0.1 to 2% by weight, relative to the layer.

Stabilizers which can be employed are the customary compounds which havea stabilizing action on ethylene polymers, propylene polymers and otherα-olefin polymers. The effective amount is, in general, about 0.1 to 2%by weight, relative to the layer.

The low-molecular resin recommended is a natural or synthetic resinhaving a softening point of about 60° to 180° C., preferably of about80° to 130° C. (determined as specified in ASTME 28). Amongst thenumerous low-molecular resins, the hydrocarbon resins are preferred,specifically in the form of petroleum resins, styrene resins,cyclopentadiene resins and terpene resins. These resins are described inUllmanns Enzyklopadie der Techn. Chemie ["Ullmann's Encyclopedia ofIndustrial Chemistry"], 4th Edition, Volume 2, pages 539 to 553.

The petroleum resins are hydrocarbon resins prepared by polymerizingdeep-decomposed petroleum materials in the presence of a catalyst. Thesepetroleum materials usually include a mixture of resin-formingsubstances, such as styrene, methylstyrene, vinyltoluene, indene,methylindene, butadiene, isoprene, piperylene and pentylene. Styreneresins are low-molecular homopolymers of styrene or copolymers ofstyrene with other monomers, such as α-methylstyrene, vinyltoluene andbutadiene. The cyclopentadiene resins are cyclopentadiene homopolymersor cyclopentadiene copolymers obtained from coal tar distillates andfractionated petroleum gas. These resins are prepared by subjecting thematerials containing cyclopentadiene to a high temperature for a verylong time. Depending on the reaction temperature, it is possible toobtain dimers, trimers or high polymers. The terpene resins are polymersof terpenes, i.e., hydrocarbons of the formula C₁₀ H₁₆, which arepresent in nearly all the essential oils and oil-containing resins ofplants, and phenol-modified terpene resins. Special examples of terpeneswhich should be mentioned are α-pinene, β-pinene, dipentene, limonene,myrcene, bornylene, camphene and similar terpenes. The hydrocarbonresins can also comprise the so-called modified hydrocarbon resins.Modification is generally effected by reacting the raw materials beforepolymerization, by introducing special monomers or by reacting thepolymerized product, the reactions carried out being, in particular,hydrogenations or partial hydrogenations.

The hydrocarbon resins employed are preferably styrene homopolymers,styrene copolymers, cyclopentadiene homopolymers, cyclopentadienecopolymers and/or terpene polymers having a softening point in each caseof about 60° to 180° C., preferably of about 80° to 130° C. In the caseof the unsaturated polymers the hydrogenated product is preferred.

The effective amount of low-molecular resin is about 3 to 15% by weight,preferably about 5 to 10% by weight, for use in the sealing layers.

Further inert additives, such as fillers or pigments, may beincorporated in the base layer in the form of evenly distributed fineparticles. The average particle size expediently is about 0.02 to 4 μm,preferably about 0.3 to 1 μm. The content of inert additives in generalis about 1 to 25% by weight, particularly about 5 to 15% by weight,relative to the base layer. The inert additives particularly comprisepulverulent inorganic additives, for example carbonates of alkali metalsand alkaline earth metals, such as calcium carbonate (chalk); sulfates,such as barium sulfate; and/or oxides, such as inorganic siliconcompounds, in particular magnesium silicate (talcum), Na-Al-silicate,silicon dioxide, glass or titanium dioxide or mixtures of the compoundsenumerated. During the extrusion of the film, these fine-particulateadditives, which are, e.g., present in the form of a powder, are workedinto the plastic melt for the base layer and dispersed in the melt. Itis particular advantageous to employ the masterbatch technique forworking the inert particles and other additives into the base layer.

In another embodiment, the inert particles comprise an organic plasticsmaterial, which is incompatible with the polymers of the base layer andthe sealing layers and has a higher melting point than the latter. Thesepolymers include styrene polymers and acryl polymers, in particularpolystyrene and polymethyl methacrylate, fluorinated hydrocarbonpolymers, polyamides and polyesters, particularly polybutyleneterephthalate. It is also possible to employ a combination of organicand inorganic inert particles.

The inert particles partially protrude from the base layer and theprotruding portions of their surfaces are completely covered by thesealing layers (or top layers). In a preferred embodiment, at least oneof the sealing layers has a thickness which is less than the averageparticle size. Such a relatively thin sealing layer is given a texturedsurface structure by the particles projecting from the base layer, whichleads to an increased roughness of the film surface. The desiredroughness value of a film surface is set by varying the concentration ofthe inert particles, the average particle size and the thickness of thetop layer. Advantageously, the average peak-to-valley height (surfaceroughness R_(z)) of the film surface is between about 0.5 and 5 μm,particularly between about 1 and 4 μm, determined according to DIN 4768,with a cut-off of about 0.25 mm.

In a preferred embodiment, the base layer has a porous structure. As aresult thereof, the actual density of the film is less than thecalculated density based on the weight of the employed mixture ofpolypropylene and inert particles. In particular, the density of thefilm is about 0.50 to 0.85 g/cm³. If the density is too low, the filmstrength is insufficient. The porous structure results frommicro-cracks, micro-cavities or vacuoles (voids) which form during thestretching of the coextruded film when solid particles are present inthe base layer. The temperature conditions for the stretching processare such that the polymer matrix of the base layer is torn at thesurface of the inert particles, which leads to the formation of thefree, unfilled spaces i.e., the above-mentioned voids or micro-cavities,in the film. An increased number of voids and hence a reduced density ofthe film can be achieved by increasing the concentration of solidparticles.

The thickness of the polyolefin multi-layer film according to thepresent invention can vary within wide limits and depends especially onthe intended use. Its total thickness is, in general, about 10 to 120μm, preferably about 20 to 80 μm, the sealing layers being in each caseabout 0.2 to 4 μm, preferably about 0.5 to 1.5 μm, thick.

The preparation of the polyolefin film according to the presentinvention, which is comprised of three layers, is effected by acoextrusion process. Within the scope of this process the procedurefollowed is to coextrude the melts corresponding to the individuallayers of the film through a flat die, to chill the film obtained bycoextrusion in order to solidify it, to stretch (orient) the filmbiaxially, to heat-set the biaxially stretched film and to subject tocorona treatment the sealing layer(s) scheduled for corona treatment.The biaxial stretching (orientation) can be carried out simultaneouslyor successively, successive biaxial stretching, in which stretching isfirst carried out longitudinally, i.e., in the machine direction, andthen transversely, i.e., perpendicularly to the direction of themachine, being preferred. Thus, as in the conventional coextrusionprocess, the polymer or the polymer mixture of the individual layers isfirst compressed or liquefied in an extruder. The melts are then forcedsimultaneously through a flat die (slot die), and the multilayer filmwhich has been forced out is chilled and solidified on one or more rollswhich are kept at about 30° to 50° C. by cooling. The film thus obtainedis then stretched longitudinally and transversely to the direction ofextrusion, which results in an orientation of the molecular chains. Itis preferable to stretch in a ratio of about 4 to 7:1 in thelongitudinal direction and to stretch in a ratio of about 8 to 10:1 inthe transverse direction. The longitudinal stretching is carried out ata film temperature of, preferably, about 120° to 140° C., and thetransverse stretching is preferably carried out at about 160° to 175° C.It will be expedient to carry out the longitudinal stretching by meansof two rollers running at different speeds, corresponding to thestretching ratio desired, and to carry out the transverse stretching bymeans of an appropriate tenter frame. After the film has been stretchedbiaxially it is heat-set (subjected to heat treatment). In this process,the film is kept at a temperature of about 150° to 160° C. for about 0.5to 10 seconds. As mentioned hereinbefore, the corona treatment ispreferably carried out by means of an alternating voltage of about10,000 volt and about 10,000 Hertz. The film prepared in this way iswound up in a customary manner by means of a winding unit. For an exactmetering of the additives, it is in most cases advantageous to employthe so-called masterbatch technology.

The polyolefin multi-layer film according to the present invention isparticularly suitable for use as a packaging film on high-speedpackaging machines. This is because it possesses all the importantproperties demanded from polyolefin films with respect to use onhigh-speed machines. In particular, it can be sealed on both sides, hasexcellent running characteristics and at the same time good printabilityand coatability.

The present invention will now be illustrated in greater detail by meansof examples:

The examples and comparison examples below relate in each case to abiaxially oriented (longitudinal stretching ratio 5:1, transversestretching ratio 10:1) polyolefin film having a base layer and twosealing layers. The base layer is comprised of an isotacticpolypropylene having a fraction soluble in n-heptane of 5% by weight, amelting point of 165° C. and a melt flow index of 3.5 g/10 minutes at230° C. and 21.6N loading (DIN 53 735). The base layer is about 28 μmthick, and the two sealing layers provided on top of the two surfaces ofthe base layer have a thickness of about 1 μm each. The three-layerpolyolefin films are prepared by the above-described coextrusionprocess.

The sealing layers are both comprised of a random ethylene/propylenecopolymer having an ethylene content of 4.5% by weight. The raw materialemployed for the two top layers has a melt flow index of 6 g/10 minutes.The anti-blocking agent included therein is CaCO₃ having an averageparticle size of 2 μm, which is added in an amount of 0.2% by weight.

EXAMPLE 1

The polymer for the base layer included 1% by weight of apolydimethylsiloxane having a viscosity of 50 mm² /s. The film was notcorona-treated.

EXAMPLE 2

Example 1 was repeated, except that one side of the film wascorona-treated.

EXAMPLE 3

Example 1 was repeated, except that both sides of the film werecorona-treated.

EXAMPLE 4

Example 1 was repeated, except that the polydimethylsiloxane having aviscosity of 50 mm² /s was replaced by a polydimethylsiloxane having aviscosity of 100 mm² /s.

EXAMPLE 5

As Example 4, but including a single-sided corona treatment.

EXAMPLE 6

As Example 4, but including a double-sided corona treatment.

EXAMPLE 7

The raw material for the base layer included 0% by weight ofpolydimethylsiloxane, the raw material for the top layers included 0.5%by weight of a polydimethlysiloxane having a viscosity of 100 mm² /s.Both sides of the film were subjected to corona treatment.

The surface tension of the corona-treated top layer(s) was 40 mN/m ineach case, determined according to DIN 53 364, with the films beingfirst immersed in n-heptane for 5 minutes and measurement beingperformed after evaporation of the solvent. Employing this method, thesurface tension of the film prepared in accordance with Example 7 couldnot be determined.

RESULTS

The films of Examples 2 to 6 were superior to the films of Examples 1and 7. They simultaneously exhibited good sealing properties and runningcharacteristics and their corona-treated surfaces were, in particular,readily printable. The film of Example 1 could not be printed, and itwas found that the sealing properties of the film of Example 7 wereinadequate and that its surface tension could not be measured employinga conventional method.

What is claimed is:
 1. A three layer, biaxially-oriented polyolefin filmhaving two exterior surfaces which can be sealed, comprising a baselayer comprised of propylene and having a first and second surface, andfirst and second sealing layers comprised of a sealable olefin polymerand disposed on said first and second surfaces, respectively, of saidbase layer, wherein said base layer includes about 0.5 to 2.0% by weightof a polydialkylsiloxane which has a viscosity of less than about 500mm² /s.
 2. A polyolefin film according to claim 1, wherein said firstsealing layer has been subjected to a corona treatment.
 3. A polyolefinfilm according to claim 1, wherein said first and second sealing layershave been subjected to a corona treatment.
 4. A polyolefin filmaccording to claim 1, wherein said polydialkylsiloxane has a viscosityof about 10 to 100 mm² /s.
 5. A polyolefin film according to claim 1,wherein said polydialkylsiloxane includes alkyl groups having 1 to 4carbon atoms.
 6. A polyolefin film according to claim 1, wherein saidpolydialkylsiloxane comprises a polydimethylsiloxane.
 7. A polyolefinfilm according to claim 1, wherein said base layer includes about 0.6 to1.5% by weight of said polydialkylsiloxane.
 8. A polyolefin filmaccording to claim 1, wherein said base layer includes about 0.7 to 1%by weight of said polydialkylsiloxane.
 9. A polyolefin film according toclaim 1, wherein said base layer comprises a polymer selected from thegroup consisting of an isotactic polypropylene having ann-heptane-soluble proportion of about 15% by weight or less, a copolymerof ethylene and propylene having an ethylene proportion of about 10% byweight or less, and a copolymer of propylene and C₄ to C₈ α-olefinhaving an α-olefin proportion of about 10% by weight or less.
 10. Apolyolefin film according to claim 1, wherein said first and secondsealing layers comprise a polymer selected from the group consisting ofan ethylene homopolymer, a copolymer of ethylene and propylene, acopolymer of ethylene and butylene, a copolymer of propylene andbutylene, a terpolymer of ethylene, propylene and butylene, and mixturesthereof.
 11. A polyolefin film according to claim 1, further comprisingat least one additive selected from the group consisting of anantistatic agent, antiblocking agent, slip agent, filler, pigment, dye,stabilizer and low-molecular resin.
 12. A polyolefin film according toclaim 2, wherein the exterior surface of said corona-treated firstsealing layer is printed with an aqueous printing ink.
 13. A polyolefinfilm according to claim 3, wherein the exterior surface of said firstsealing layer is printed with an aqueous printing ink and the exteriorsurface of said second sealing layer is bonded to a sheet-like substrateby means of an aqueous adhesive.
 14. A packaging film comprising apolyolefin film according to claim 1.